Handover method, and a cellular radio system

ABSTRACT

A cellular radio system and a handover method in a cellular radio system is provided. The cellular radio system includes a subscriber terminal and a base station. In the system, the subscriber terminal maintains and continuously updates an active set of base stations. One or several base stations belonging to the active set has a connection or connection set-up facilities to the subscriber terminal. The subscriber terminal controls the downlink transmission of one or several base stations of the active set by switching the transmission of the base station on or off. This method provides a fast soft handover similar to a hard handover.

This application is the national phase of international applicationPCT/F195/00467 filed Aug. 31, 1995 which designated the U.S.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a handover method in a cellular radio systemcomprising at least one subscriber terminal and a base station, and inwhich system the subscriber terminal maintains and continuously updatesan active set of base stations, one or several base stations belongingto the active set having a connection or connection set-up facilities tothe subscriber terminal.

The invention also relates to a cellular radio system comprising atleast one subscriber terminal and a base station, in which system thesubscriber terminal maintains and continuously updates an active set ofbase stations, one or several base stations belonging to the active sethaving a connection or connection set-up facilities to the subscriberterminal.

2. Description of Related Art

When a subscriber terminal moves from one cell to another in a cellularradio system, a handover is performed, and this handover is either, in asimple case, a hard handover or a more flexible soft handover. Thedrawback of the hard handover is that the old connection is brokenbefore a new one is set up. The problem is more marked especially in theping-ponging effect wherein a channel keeps changing back and forthbetween different channels. This can be diminished by using a handovermargin. However, the best base station connection cannot be utilized insuch a case due to the handover margin and the delay of the handoverprocedure, wherefore an unnecessarily high transmit power must be usedand the disturbance power of the system thereby increases.

The prior art soft handover also employs updating margins, which arecovered, however, in the reception of the subscriber terminal by signalsfrom base stations with better audibility. Furthermore, even theseunnecessary signals increase the transmit power of the base stations andinterfere with the other subscriber terminals. The prior art handover isdescribed for example in Finnish Patent Application 952,396 (Granlund,Häkkinen, Hämäläinen): “Method for improving the reliability of handoverand call establishment, and a cellular radio system”, which isincorporated herein by reference.

As it is known, a pilot signal is used to identify a base station and toform an active set in the CDMA system. A pilot signal is adata-unmodulated spreading-coded signal, which is continuouslytransmitted by each base station to its coverage area. A terminalequipment can identify the base stations on the basis of the pilotsignal, since the spreading codes of the pilot signals differ from oneanother.

Subscriber terminals continuously measure pilot signals. In order toreduce the measurement load of a terminal equipment in prior artsystems, each terminal equipment maintains a measurement list of thebase stations and the corresponding spreading codes of the pilot signalsthat are situated near the terminal equipment and that are possiblecandidates for handover or connection establishment. The base stationson the measurement list form a group of candidates, which may becomemembers of the active set. It is possible to establish connectionsrapidly to the active set from the fixed network. Terminal equipmentsmonitor with the highest priority the pilot signals of only those basestations that are on the measurement list.

When a terminal equipment moves, the measurement list must naturally beupdated as the need arises. In the prior art systems updating isperformed according to the measurement performed by the terminalequipment on the strength of the pilot signal, i.e. if a pilottransmitted by a base station is received with adequate strength, it isadded to the measurement list.

A rake receiver used in particular in the CDMA system comprises severalbranches, each of which may be synchronized with a different signalcomponent. The receiver can therefore receive several signalssimultaneously. On the basis of the measurements of the pilot signal,the branches of the rake receiver are also caused to receive signalsthat arrive along different propagation paths. The rake receiver adaptsto the attenuation changes over the different connections considerablyfaster than the active set is updated.

In a CDMA network providing many services there occur, however,situations wherein the load of the base station in the transmissiondirection from the terminal equipment to the base station, i.e. in theuplink transmission direction, is considerably greater than in theopposite transmission direction. An example of this is theunidirectional data transmission from the terminal equipment to thenetwork. The prior art arrangements for updating the measurement list donot detect and therefore change the load of the base station in thistransmission direction.

Even though the best signal can be selected and the power of signalswhich have propagated along different paths can be compiled when a rakereceiver is used in the subscriber terminal, the number of the rakebranches is limited by the power consumption and the manufacturingcosts, and therefore the rake receiver cannot utilize, however, morethan a few transmissions at a time.

Neither do the known systems provide the possibility of transferringconnections to other base stations in a situation where an individualbase station is overloaded.

SUMMARY OF THE INVENTION

The purpose of the present invention is to avoid the problems of theknown arrangements and to implement a soft handover in the manner ofhard handover with a very small effective handover margin, and todecrease the disturbance power of the base station when the subscriberterminal communicates with several base stations.

This is achieved with the method of the type described in the preamble,characterized in that the subscriber terminal controls the downlinktransmission of one or several base stations of the active set in such away that each base station switches its transmission on or off.

The cellular radio system according to the invention is characterized inthat the subscriber terminal comprises means for controlling one orseveral base stations of the active set in such a way that each basestation switches its downlink transmission on or off.

The invention provides considerable advantages. The number of signalstransmitted by the base stations can be decreased, and the interferencelevel caused by the base stations can thereby be reduced. This in turnincreases the capacity of the system and improves the quality of theconnections.

Handover also becomes faster. The updating of the active set is slow andrequires a great deal of signalling. According to the invention, a basestation can be activated by means of the internal signalling of theradio interface or alternatively by means of routed signalling. A mobilestation has to keep active only 1 to 3 base stations providing the bestconnection, and to update this group rapidly by means of the signallingaccording to the invention. The result is a process, similar to a hardhandover, from the base station to the terminal equipment with a verysmall effective handover margin.

The fixed network may operate in the same manner as in a conventionalmacro diversity system, and the arrangements according to the inventionare only directed at the radio interface.

The invention enables the size of the active set and the selectionmargin to be increased without increasing the interference. Thisfacilitates the maintenance of the best connection especially underdifficult circumstances in a microcell environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below with referenceto the examples according to the accompanying drawings, in which

FIG. 1 shows a cellular radio system,

FIG. 2 shows the essential parts of a subscriber terminal, and

FIG. 3 shows the essential parts of a base station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method according to the invention will be described in greaterdetail below. In the method according to the invention, the selection ofthe macro diversity active set is based on the comparison of pilotsignals, transmitted by the base stations, in the subscriber terminal.When the power of a pilot signal of a base station received at asubscriber terminal comes close to the power of the strongest receivedpilot signal within a certain margin, this base station is added to theactive set of base stations maintained by the subscriber terminal.Otherwise the base station is removed from the active set when itssignal deteriorates. The base stations of the active set aresynchronized and receive the signal of the subscriber terminal if thesignal-to-interference ratio enables it. Connections are establishedfrom the base stations through the fixed network, and through theseconnections the signals received by the base stations can be combined.The signal of the base stations of the active set to be transmitted tothe subscriber terminal is transferred in a similar manner. The basestations of the active set have connections or at least connectionset-up facilities to the subscriber terminals.

By means of measurements of signal quality, a subscriber terminal canchange the transmission of the base stations. The subscriber terminalcontrols the downlink transmission of the base stations by switching thetransmission on or off. For example, the subscriber terminal can decideto switch on or off the transmission of a base station actively andautonomously. The subscriber terminal transmits a command signal thatorders at least one base station of the active set to switch itsdownlink transmission on or off. When the base station transmission isswitched off in the arrangement according to the invention, the basestation transmission is completely terminated or the transmit power isdecreased by a desired degree, so that the interference caused by thebase station disappears. The decrease in the transmit power can be forexample between 40 dB and 3 dB. Switching a base station transmission onand off is preferably performed by means of a command signal, which canbe received by the base station directly from the transmission of thesubscriber terminal, or the command signal can be forwarded by the fixednetwork. Especially the command signal that switches on the base stationtransmission is either common to all the base stations of the activeset, or it is separate for each base station, and this command signal istransmitted regularly, preferably at intervals of a frame. Transmittingthe command signal regularly provides the advantage that the transmitpower of the base stations can be adjusted at precise intervals, andwhen the interval between the adjustments is short, for example oneframe, the adjustment is performed rapidly enough to observe even fastchanges in the attenuation over the connection. All base stationsreceiving the command signal perform the power adjustment. The basestations which cannot receive the signal of the subscriber terminalobserve the transmission error and terminate the transmission to thissubscriber terminal. The transmission is terminated most preferably bydecreasing the transmit power slowly within a predetermined period oftime or by disconnecting the transmission after a predetermined delay.When the base station terminates the transmission after it has lost theconnection, i.e. after the connection has deteriorated below apredetermined level, the advantage is that the number of the basestations communicating with the subscriber terminal can be restricted tothe smallest possible. The base station starts retransmitting when itcan receive the command signal of the subscriber terminal switching onthe base station.

In another preferred embodiment of the invention, the subscriberterminal transmits a command signal by means of which the transmissionof each base station of the active set is separately adjusted. Each ofthe base stations can separately receive the command signal, so thateach base station adjusts its own transmit power according to what thecommand signal determines. The base stations that do not receive thecontrol signal terminate the transmission to the subscriber terminal inthe same manner as in the first preferred embodiment. The advantage oftransmitting the command signal separately for each base station is thatthe method is then more controlled and the power level of each basestation can be separately adjusted to the desired value. The commandsignal can also be assembled of signals received by several basestations. The control signal is then assembled in some part of thenetwork, for example in the base station controller, and the controlsignals are signalled separately to each base station.

FIG. 1 is a diagram of a cellular radio system. The arrangementaccording to the invention is applicable in all interference-limitedcellular radio systems, which include for example different spreadspectrum systems, OFDMA systems, and in the preferred embodiment of theinvention, the CDMA system. The cellular radio system comprises a numberof subscriber terminals 101 and 102, and base stations 103 to 107. Thesubscriber terminals 101 and 102 communicate with the base stations 103to 107 on certain traffic channels when the subscriber terminals aresituated within the coverage area of the base stations. When the CDMAsystem is used, a traffic channel consists of a wide frequency band usedby all terminal equipments 101 and 102 as they transmit to the basestation 103 to 107 and also used by the base stations 103 to 107 in thedownlink transmission direction. It is assumed in FIG. 1 that thesubscriber terminal 101 communicates with the base stations 103 and 104,but that it is in the process of shifting the connection from the basestation 104 to the base station 105, since the connection with the basestation 104 deteriorates and the connection with the base station 105 isimproved. The terminal equipment 101 transmits a command signal 110,which connects the base stations to transmit in the downlinktransmission direction. In this example, the base stations 103, 104 and105 form the active set of the subscriber terminal 101, since thequality of the connection to these base stations is better than therequired signal noise level. Since the connection to the base station105 is improving, which can be seen on the basis of the measurement onthe pilot signal, the base station 105 then receives the control andcontinues the connection with the terminal equipment 101. The connectionto the base station 104 deteriorates further and the base stationfinally loses the command signal 110. According to the arrangement ofthe invention, the base station then terminates the connection to theterminal equipment 101. The subscriber terminal may also transmit acommand signal by means of which the downlink transmission of the basestation is switched off. This command signal is preferably transmittedseparately for each base station, so that not all the base stations ofthe active set terminate their transmission.

A base station controller 108 manages the control of the base stations103 to 106 and assembles the signals arriving at the base stations 103to 106 when the different parts of the signal have been received atdifferent base stations. The base station controller 108 operates inthis manner especially when, in an arrangement of the invention, thesubscriber terminal 101 transmits the command signal 110 regulating thetransmit power of the base stations in such a way that the differentparts of the signal are received at different base stations. The basestation controller 108 forwards the message of the command signal it hasassembled via the fixed network 109 to the base stations 103 to 106,which operate according to the command of the control signal 110.

FIG. 2 illustrates a transceiver of a subscriber terminal in thecellular radio system, the method according to the invention beingapplicable in such a transceiver. The receiving part of the subscriberterminal comprises a receiver unit 216 receiving a signal that isconverted to an intermediate frequency, and converter means 218 in whichthe signal is converted to a digital form. The converted signal issupplied to detector means 220 from where the detected signal issupplied further to de-interleaving means 222 and to means 224 whereinthe received signal is decoded, i.e. subjected to both channel andspeech decoding.

The transmitting part of the subscriber terminal further comprises means200 for encoding the signal to be transmitted. The encoded signal issupplied to means 202 for interleaving the encoded signal. The outputsignal of the interleaving means is connected to the input of the means204, where the burst to be transmitted is formed. The resulting signalis supplied to modulation means 208, the output signal of which issupplied via a transmitter unit 210 and a duplex filter 212 to anantenna 214. The aforementioned blocks can be implemented in knownmanners.

The apparatus further comprises control and counting means 226, whichcontrol the operation of the other aforementioned blocks. The controlmeans 226 may be used to adjust the transmit power of the subscriberterminal itself, i.e. the signalling and traffic channels of thesubscriber terminal can be adjusted. In the arrangement according to theinvention this concerns especially the command signal adjusting thetransmit power of the base stations. When the power of the commandsignal is regulated, the number of the base stations situated within thecoverage area of the subscriber terminal can be restricted. Thisprovides the advantage that the subscriber terminal can keep active only1 to 3 base stations providing the best connection, and update thisgroup rapidly by means of signalling according to the invention. Thisresults advantageously in operation similar to a hard handover in thedownlink transmission direction with a very small effective handovermargin. The control means 226 receive and compares signals fromdifferent base stations. On the basis of the comparison, means 226decides whether to switch on or off a downlink transmission of at leastone base station of the active set. Means 226 a comprised by the commandunit forms a command signal or control signals adjusting the transmitpower of the base stations, the signal(s) being transmitted normally viathe transmitting part. Adjusting the transmission of the base stationsprovides the advantage that the interference level caused by the basestations can be reduced, since the total transmit power of several basestations can be optimized to suit each situation. In the arrangementaccording to the invention, the transmission of the base stations isadjusted in such a way that the control signal switches the base stationtransmission on or off. When the base station transmission isdisconnected, its transmit power is entirely switched off or thetransmit power has been reduced by a certain amount, for example 20 dB.The advantage provided by this procedure is a fast handover. The controlmeans 226 can also be used to adjust the transmit power of the terminalequipment itself, the advantage being that the number of the basestations with which the terminal equipment communicates can berestricted and the number of the interference signals can besimultaneously reduced. The control means 226 and 226 a of thesubscriber terminal are typically realized by means of a processor, butthey may also be realized with several other kinds of electronicconnections, which may perform similar functions as a processor. Thesubscriber terminal also comprises means 220, 226 for measuring thestrength of a pilot signal it has received from a base station. If thesystem is a CDMA system, the detector block in the receivers, usuallyrealized according to the rake principle, typically comprises severalreceiver branches at least one of which is a so-called searcher branch,which measures the strengths of the pilot signals.

FIG. 3 is a diagram illustrating a transceiver of a base station. Thetransceiver of the base station is almost identical to the transceiverof the subscriber terminal. The transceiver of the cellular radio systemaccording to the invention comprises means 220, 226 for estimating theattenuation over the connection between the terminal equipment and thebase station by means of the received power of the pilot signal and thepower used in the transmission. The transceiver of the base stationfurther comprises means 220, 226 for estimating the signal-to-noiseratio of the uplink transmission direction on the basis of the totalinterference, the attenuation over the connection, and the transmitpower of the terminal equipment. The base station equipment alsocomprises means 220, 226 for measuring the total interference of thesignal it has received from the terminal equipments, and means 220 to214, 226 for transmitting a pilot signal with known transmit power.

The greatest difference between the arrangement according to theinvention and the prior art is in the control means 226. The controlmeans 226 of the base station transceiver in the arrangement of theinvention comprises means 226 b for initiating a transmission to thesubscriber terminal when the base station identifies the command signalof the subscriber terminal, and with the same means 226 b the basestation terminates the transmission to the subscriber terminal when thebase station no longer identifies the command signal transmitted by thesubscriber terminal to switch on the transmission, or when the basestation receives a command signal by means of which the transmission isswitched off. It is possible to terminate the transmission slowly withthe means 226 b and to avoid an immediate disconnection by reducing thetransmit power of the base station within a predetermined delay.Terminating the transmission when the command signal is no longer clearreduces the number of the base stations communicating with a particularsubscriber terminal, thus decreasing interference. Avoiding an immediateconnection breakdown provides the advantage that the connection betweenthe base station and the subscriber terminal is not lost due to atemporary deterioration in the signal-to-interference ratio. The controlmeans 226 and 226 b of the base station are typically realized by meansof a processor, but they may also be realized with several other kindsof electronic connections, which may perform similar functions as aprocessor.

Even though the invention is described above with reference to theexample according to the accompanying drawings, it is clear that theinvention is not restricted thereto, but it can be modified in many waysaccording to the inventive idea disclosed in the appended claims.

What is claimed is:
 1. A handover method in a cellular radio systemincluding a subscriber terminal and a plurality of base stations, themethod comprising: maintaining and continuously updating, by thesubscriber terminal, an active set of the base stations, at least one ofthe base stations belonging to the active set having a respectiveconnection to the subscriber terminal; and deciding, by the subscriberterminal, to switch on or off a respective downlink transmission of theat least one base station of the active set.
 2. A method according toclaim 1, wherein the subscriber terminal transmits a command signalordering the at least one base station of the active set to switch onthe respective downlink transmission.
 3. A method according to claim 1,wherein the subscriber terminal transmits a command signal ordering theat least one base station of the active set to switch off the respectivedownlink transmission.
 4. A method according to claim 1, wherein thesubscriber terminal decides to switch on or off the respective downlinktransmission of the at least one base station of the active set bycontrolling signaling and a traffic channel in such a way that the atleast one base station of the active set loses the respective connectionto the subscriber terminal, whereupon the at least one base station ofthe active set switches off the respective downlink transmission.
 5. Amethod according to claim 1, wherein the at least one base station ofthe active set switches off the respective connection with thesubscriber terminal after a predetermined delay.
 6. A method accordingto claim 1, wherein the at least one base station of the active setswitches off the respective connection with the subscriber terminal byreducing transmit power within a predetermined period of time.
 7. Amethod according to claim 2, wherein the subscriber terminal transmits acommand signal separately for each base station.
 8. A method accordingto claim 2, wherein the command signal is common to all base stations ofthe active set.
 9. A method according to claim 2, wherein the subscriberterminal transmits a command signal at regular intervals.
 10. A methodaccording to claim 9, wherein a transmission of traffic channels takesplace at regular intervals in a form of frames and the subscriberterminal transmits the command signal at intervals of a frame.
 11. Amethod according to claim 2, wherein when the command signal intendedfor the at least one base station of the active set is divided infragments received by different base stations, the command signalintended for the at least one base station is assembled in a basestation controller or in some part of the network from said fragments,and the command signal formed in this manner is signaled back to the atleast one base station.
 12. A cellular radio system comprising: asubscriber terminal; and a plurality of base stations, the subscriberterminal maintaining and continuously updating an active set of the basestations, at least one of the base stations belonging to the active setand having a connection to the subscriber terminal, wherein thesubscriber terminal comprises: means for deciding to switch on or off adownlink transmission of at least one base station of the active set.13. A cellular radio system according to claim 12, wherein thesubscriber terminal comprises means for forming and transmitting acommand signal ordering the at least one base station of the active setto switch on a transmission.
 14. A cellular radio system according toclaim 12, wherein the subscriber terminal comprises means for formingand transmitting a command signal ordering the at least one base stationof the active set to switch off a transmission.
 15. A cellular radiosystem according to claim 12, wherein the means for deciding to switchon or off downlink transmission of at least one base station of theactive set is arranged to control signaling and traffic channels of thesubscriber terminal in such a way that the at least one base station ofthe active set loses the connection to the subscriber terminal, and atleast one of the base stations comprises means for switching off arespective downlink transmission when the at least one of the basestation loses a respective connection to the subscriber terminal.
 16. Acellular radio system according to claim 12, wherein the base stations,each comprise means for switching off a respective transmission after apredetermined delay.
 17. A cellular radio system according to claim 12,wherein the base stations, each comprise means for switching off arespective transmission by reducing transmit power within apredetermined period of time.
 18. A cellular radio system according toclaim 13, wherein the subscriber terminal comprises means fortransmitting a command signal separately for each of the base stations.19. A cellular radio system according to claim 13, wherein thesubscriber terminal comprises means for transmitting a command signalthat is common to all of the base stations of the active set.
 20. Acellular radio system according to claim 12, wherein the subscriberterminal comprises means for transmitting a command signal at regularintervals.
 21. A cellular radio system according to claim 20, whereintransmission of traffic channels takes place at regular intervals in aform of frames and the subscriber terminal comprises means fortransmitting a command signal at intervals of a frame.
 22. A subscriberterminal for a cellular radio system, wherein the subscriber terminalcomprises: means for maintaining and continually updating an active setof base stations; and means for deciding to switch on or off arespective downlink of at least one of the base stations of the activeset.
 23. The subscriber terminal according to claim 22, wherein saidmeans for deciding to switch on or off a a respective downlink of atleast one of the base stations of the active set comprises means fortransmitting a command signal to order the at least one base station ofthe active set to switch on the respective downlink transmission. 24.The subscriber terminal according to claim 22, wherein said means fordeciding to switch on or off a a respective downlink of at least one ofthe base stations of the active set comprises means for transmitting acommand signal to order the at least one base station of the active setto switch off the respective downlink transmission.
 25. The subscriberterminal according to claim 22, wherein said means for deciding toswitch on or off a a respective downlink of at least one of the basestations of the active set comprises means for controlling signaling anda traffic channel such that the at least one base station of the activeset loses a respective connection to the subscriber terminal and the atleast one base station of the active set switches off the respectivedownlink transmission.
 26. A method according to claim 3, wherein the atleast one base station of the active set switches off the respectiveconnection with the subscriber terminal after a predetermined delay. 27.A method according to claim 4, wherein the at least one base station ofthe active set switches off the respective connection with thesubscriber terminal after a predetermined delay.
 28. A method accordingto claim 3 wherein the at least one base station of the active setswitches off the respective connection with the subscriber terminal byreducing transmit power within a predetermined period of time.
 29. Amethod according to claim 4 wherein the at least one base station of theactive set switches off the respective connection with the subscriberterminal by reducing transmit power within a predetermined period oftime.
 30. A method according to claim 3, wherein the subscriber terminaltransmits a command signal separately for each base station.
 31. Amethod according to claim 3, wherein when the command signal intendedfor the at least one base station of the active set is divided infragments received by different base stations, the command signalintended for the at least one base station is assembled in a basestation controller or in some part of the network from said fragments,and the command signal formed in this manner is signaled back to the atleast one base station.
 32. A cellular radio system according to claim14, wherein the base stations, each comprise means for switching off arespective transmission after a predetermined delay.
 33. A cellularradio system according to claim 15, wherein the base stations, eachcomprise means for switching off a respective transmission after apredetermined delay.
 34. A cellular radio system according to claim 14,wherein the base stations, each comprise means for switching off arespective transmission by reducing transmit power within apredetermined period of time.
 35. A cellular radio system according toclaim 15, wherein the base stations, each comprise means for switchingoff a respective transmission by reducing transmit power within apredetermined period of time.
 36. A cellular radio system according toclaim 14, wherein the subscriber terminal comprises means fortransmitting a command signal separately for each of the base stations.37. A cellular radio system according to claim 14, wherein thesubscriber terminal comprises means for transmitting a command signalthat is common to all of the base stations of the active set.